Method for making honeycomb panel



n 1963 H. R. HERMAN ETAL 3,091,845

METHOD FOR MAKING HONEYCOMB PANEL 6 Sheets-Sheet 1 INVENTORS. H. R.HERMAN B. R. ALSOBROOK Original Filed Nov. 26, 1954 ATTORNEY June 4,1963 H. R. HERMAN ETAL 3,091,845

METHOD FOR MAKING HONEYCOMB PANEL Original Filed Nov. 26, 1954 6Sheets-Sheet 2 INVENTORS. H. R. HERMAN BY B. R. ALSOBROOK M 2. -W4MmATTORNEY June 4, 1963 H. R. HERMAN ETAL METHOD FOR MAKING HONEYCOMBPANEL Original Filed Nov. 26, 1954 6 Sheets-Sheet 3 IN VENT'ORS.

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METHOD FOR MAKING HONEYCOMB PANEL 6 Sheets-Sheet 4 Original Filed Nov.26, 1954 m m N w m H. R. HERMAN R. ALSOBROOK ATTORNEY June 1963 H. R.HERMAN ETAL 3,

METHOD FOR MAKING HONEYCOMB PANEL Or iginal Filed Nov. 26, 1954 6Sheets-Sheet 5 INVENTORS- H. R. HERMAN B. R. ALSOBROOK ME- W mu mn I .mwwl 2 O O m MOHQL ATTORNEY June 1963 H. R. HERMAN EfAl. 3,091,845

METHOD FOR MAKING HONEYCOMB PANEL Original Filed Nov. 26, 1954 6Sheets-Sheet 6 HUNG 1/4 96 m 1/3 In, 6 /12 I 1 i HI w il fiflllml. 977 m\VACUUM He /2 SOURCE .HERMAN BY .ALSOBROOK Z'Y24AA M ATTORNEY StatesThis invention relates to a panel structure of sandwich type in whichthin metal shaped somewhat like a honeycomb is secured between twoenclosing skins and to the apparatus and method used for fabricatingsuch panel.

This application is a division of our copending application for Fixturefor Making Honeycomb Panel, Serial No. 471,394, filed November 26, 1954,now Patent No. 2,944,504.

An object of the invention is to provide a fixture for holding thesandwich structure while its components are being brazed together whichis capable of heating and cooling with the sandwich with a minimumamount of distortion.

Another object is to provide a holding fixture made up of a number ofsmall individual components so connected together that it will remainrigid and hold its contour even when repeatedly heated to a temperatureof about 2200 F. A further object is to provide such a fixture havingthe general form of the usual cardboard partition used to pack eggs inegg crates, and to provide for the free cir culation of gas through theindividual strips making up the lattice work so that the sandwich may beheated substantially uniformly throughout.

Another object is to provide such a lattice structure which, Whilemaintaining the necessary rigidity, .is yet of small mass and heatcapacity so that it requires only a small amount of heat to be suppliedto it to raise its temperature to that required for brazing the sandwichcomponents together.

Another object is to provide means for stiffening the lattice structureand preventing one portion of it from shifting with reference to otherportions in addition" to the rigidity provided by the contact of thelattice strips with each other. This additional stiifeningmeans ispreferably perforated to permit the free flow of gas therethrough forsecuring a more uniform heating.

A further object is to provide for a small amount of slippage of thesandwich with reference to the fixture by means of a thin sheetinterposed between them, thereby preventing warpage and change in thesandwich contour as it cools after brazing. I

Another object is to provide means between the supporting lattice andsandwich to prevent the concentration of pressure in narrow areas of thesandwich and assure a more uniform pressure distribution sandwich beforeand during the brazing.

In cases where the sandwich has a convex contour, a further object is toprovide a thin metal sheet enveloping atcnt over the the sandwich andarranged to apply substantially uniform pressure to the various portionsthereof, the thickness and material of the sheet preferably being suchthat the sheet cools at substantially the same rate as the sandwich.

Another object is to provide for the heating and brazing of the metalsandwich in a reducing or inert gas which will not form undesirablecompounds of the metals such as those due to oxidation or carburization.

A still further object is to provide for the preheating of the sandwichassembly prior to brazing in a muflie not much larger than it to atemperature somewhat below 3,091,845, Patented June 4, 1963 ice thebrazing temperature and to complete the brazing in a high temperaturefurnace in which the mufile and sandwich are placed, the furnace beingheated by a suitable heating means to a temperature above the brazingtemperature.

Another object is to provide such a muffie which will distribute theheat received from the source of furnace heat and prevent the heat raysfrom such source from directly impinging on the sandwich and therebycausing uneven heating thereof.

A further object is to provide for the slow, uniform cooling of thebrazed sandwich by removing the mufiie from the furnace and permittingthe sandwich to remain in the mufiie while cooling. This procedureprevents the cool ambient air from reaching the sandwich with resultantuneven cooling thereof.

Another object is to provide means for supporting the muffie out ofcontact with the furnace to permit free circulation of the furnace gasesaround the muflle thereby insuring its more even heating than if itrested on the furnace floor.

A further object is to provide a clamping structure for drawing anenveloping sheet against the sandwich in which an increase in clampingpressure is effected due to the difference in the coefficients ofexpansion of components of the clamping device. This arrangementcompensates for the expansion of the enveloping sheet due to the largerise in temperature and holds it firm against the sandwich.

Further objects will become apparent as the descrip tion proceeds. For abetter understanding of the invention reference is made to theaccompanying drawings, in which:

FIG. 1 is a perspective view of a honeycomb sandwich embodying theinvention;

FIG. 2 is a perspective view of a holding fixture containing thesandwich of FIG. 1; 7

FIGS. 3 and 4 are views of lattice strips used in the fixture of FIG. 2;

FIG. 5 is a view partly in seciton and partly diagramma-tic of ahoneycomb sandwich-and muffle in a furnace; 7 FIG 6- is a top view of aholding fixture illustrating a modified form of the invention;

FIG. 7 is an end view of the fixture of FIG. 6; FIG. 8 is a detail ofFIG. 6; t FIG. 9 is a sectional view on line 9-9 of FIG. 6 also showinga honeycomb panel in the fixture;

FIG. 10 is a perspective view of the sandwich panel shown in FIG. 9;

FIG. 11 is a perspective view showing a holding fixture containing asandwich panel and;

FIG. 12 is a sectional view on line 1212 of FIG. 11.

Referring to FIGS. 1 and 2 the invention is illustrated in connectionwith the construction of a honeycomb sandwith 15 having a central corecomposed of diamond shaped cells 16 which are connected together bywelding or other known means, the outer ends of the cells being brazedto a covering skin 17 and their inner ends brazed to a concave shapedskin 18, the 18 terminating in straight edges 19 to which are brazed theends of a pain of inwardly curved flanges 20, 21. The lower ends 22 ofskin 17 are also straight and have brazed thereto outwardly curvedflanges 23, 24 of the shape shown. All the above parts are preferablymade of stainless steel which has high tensile strength and although thethickness of the metal composing the honeycomb cells may be only from 1to 3-thousandths of an inch, nevertheless when the parts of the sandwichare brazed together the resulting structure has great strength andstrongly resists torsion and bending. Prior to brazing, in a manner tobe later described, the flanges 20, 21 are tack welded at spaced apartpoints to edges 19 and flanges 23, 24 simi- 3 larly secured to the edges22 of skin 17. As shown in FIG. 1A, the honeycomb is preferably providedwith small holes 25 through the metal thereby providing passages throughwhich gas may pass from one cell to another. The holes 25 thus assurethat the pressure within all the cells is substantially the same.

The brazing together of the aforementioned parts is done in a fixture 26of the type shown in FIG. 2, which comprises a horizontal base 27 in theform of a. lattice composed of a plurality of longitudinal strips 28interlocked with a plurality of transverse strips 29. Each strip 28 hasa series of equally spaced apart slots 30 and a set of holes 31therethrough and each strip 29 has a series of equally spaced apartslots 32 and a set of holes 33 therethr-ough, as shown in FIGS. 3 and 4.By assembling the strips together so that the unslotted portion of eachlongitudinal strip 28 fits into the slots of the transverse strips 29,in a manner similar to the cardboard partitions used to pack eggs in eggcrates, a rigid lattice 27 is provided which contains a relatively smallamount of metal, is light in weight and may easily be assembled andtaken apart. For certain purposes it is desirable to further stiifen thelattice by securing its component strips together at their points ofintersection 'by welding or other means (not shown). If the top of thelattice is not plane Within the desired tolerance, it may be made flatwithin a tolerance of .001 inch by grinding. The top face of the latticeserves as a horizontal reference or datum plane of the fixture. Thestrips 28 and 29 are preferably composed of a steel alloy capable ofstanding high temperatures without appreciable distortion and may beabout .062 inch thick when the strips composing the lattice are 1 inchapart. The holes 31, 33 permit hot gas to circulate through the latticestructure promoting its more rapid heating and more uniform temperature.Attached to the bottom of the lattice are a set of angle irons 34, thecenter only of each angle being preferably welded to the lattice. Theseangles support the lattice while the fixture is heated as describedbelow and permit the lattice to freely expand and contract unrestrainedby the angles which may expand at a different rate than the lattice sothat buckling of the lattice would result if it were secured to bothends of the angles. The free space between the angles and latticepermits the free circulation of hot gases therethrough to assist in therapid heating of the fixture and a more uniform temperaturedistribution.

A plurality of similar formers 35 have fiat edges 36 seated on lattice27, each former preferably being composed of a plurality of annularsegments secured together by bolts 37, as shown. Deep slots '38 are cutin the periphery of each former and longitudinally extending strips 39are seated in these slots so that the several formers are in alignmentand parallel to each other. Each former is secured in place to lattice27 by bolts 40 which pass through a pair of angle irons 41 which arewelded or otherwise secured to lattice 27. Near its base each formerterminates in a vertical edge 42, against which rests the vertical edgeof former extension 43 whose upper end 44 engages the end of thehoneycomb sandwich and whose curved edges 45, 46 fit against flanges 20,24 respectively and flanges 21, 23. After assembling the formers 35 andlattice strips 39, if their outer edges do not exactly conform to thedesired shape, the outside periphery of the assembly i ground orotherwise machined to shape.

Before assembling the sandwich components shown in FIG. 1, the insideface skin 17 and outer face of skin 18 are covered with brazing materialas are also the edges 19 and 22 of the four channel members. The brazingmaterial may be of any known type which is compatible with the metals tobe brazed together and, in the illustrated example, may be a thincoating of acrylic resin applied by a spray gun. The metal brazing alloymay then be applied as a thin sheet or as a fine powder dusted orsprayed onto the resin coating. Before placing the 4 assembled preparedsandwich in the brazing fixture, a stainless steel sheet 50 having athickness of about .020" is laid on the curved periphery of the fixtureto cover all the formers and strips 39 and the assembled sandwich thenplaced in position, as shown in FIG. 2. The former extensions 43 arethen put in place and an enveloping sheet 51 of stainless steel drapedover the sandwich and the flanges 23, 24. Sheet 51 preferably is between.030 and .040" thick and made of stainless steel. Elongated clamp jaws52 are pressed against the lower ends of sheet 51 by means of a seriesof bolts or screws 53 threaded in spaced apart similar supports 54. Theface of jaw 52 which presses against sheet 51 is serrated or otherwiseroughened to give a better grip and is contoured to the shape of flange24. Each support 54 is attached to lattice base 27 preferably by a bolt(not shown) which passes through the lattice and enters a threaded holein the bottom of the support. All the supports together with theiradjusting screws are thus readily removed from the lattice. Supports 54may be made of Invar alloy and screws 53 of metal which has asubstantially higher coeificient of expansion than lnvar so that as thetemperature increases during the brazing, the screws exert a somewhatgreater pressure on jaws 52 which in turn keep enveloping sheet 51tightly pressed against the outer skin 17 of the sandwichnotwithstanding the increase in length of sheet 51 between jaws 52 dueto increase in temperature.

After sheet 51 has been drawn tight to conform the sandwich 15 to theperipheral contour of the lattice comprising the formers 35 andstrips39, the assembly is placed in a stainless steel enclosing mufile55 (FIG. 5) which is substantially airtight, with the angles 34 restingon the floor 56 of the muffle. A stream of argon gas is then passed intothe muffie to purge it of air and a stream of "hydrogen gas then passedinto it until the mufile and honeycomb are completely filled withhydrogen. The sandwich and mufile are then preheated by any suitableheating means to a temperature of about 500 F., the heater illustratedconsisting of two Calrod electric heaters 57 supplied with current fromthe usual A.C. source (not shown). It will be understood that more thantwo heaters-may be used, if desired. This heating partly purges themuflle and assures the removal of "all moisture therefrom.

After preheating in the muffle, the muffle and sandwich are placed in ahigh temperature furnace 58, the muffie being supported above the floor59 of the furnace by a plurality of angle irons 60 arranged to provideopen spaces 61 through which the furnace gases may circulate toaccomplish more uniform heating. The furnace walls are preferably linedwith fire brick (not shown) in a known manner. The furnace may be heatedby a plurality of electric heaters 62 diagrammatically shown or by hotcombustion gases supplied from a burner (not shown), the temperature inthe furnace being maintained from to 200 F. above the melting point ofthe brazing alloy. An inert gas as, for example, hydrogen which will notoxidize or otherwise form compounds with the metal of sandwich 15 isintroduced into a metal heating coil 63 in the furnace, the outlet ofthe coil discharging the hot gas against a baffle 64 in the muffle, thebattle having discharge outlets 65 which serve to direct the hot inertgas along the wall of the muflle and thus prevent it from impingingdirectly on the sandwich with resultant uneven heating thereof.

As soon as the furnace door (not shown) is closed, the inert gas ispassed through coil 63 to heat it, the gas leaving through a pipe 66.Soon after the sandwich and interior of the muffle have reached thebrazing temperature, as indicated by one or more pyrometers (not shown)having their terminals in the mufile, the gas supply to pipe 63 is shutoff and the muflle removed from the furnace and allowed to cool in theambient air with the fixture 26 and the brazed sandwich therein. Whilecooling, another hydrogen supply is connected to supply gas into themufie against bafile 64, the hydrogen being supplied until the sandwichcools down to about 800 F., and serving to prevent oxidation of thesandwich. The hydrogen supply is then stopped and argon suppliedinstead, argon being admitted to the :muflle until it has reached normalroom temperature. The argon supply is then shut off and the cooledfixture and sandwich removed from the mufiie.

While one muille is cooling, another preheated one with its containedsandwich may be placed in the furnace and the brazing effected in themanner described. The furnace is thus fully utilized and it is notnecessary to fill it with inert gas while the brazing is effected. Alsoa further saving in this gas is secured by making the muflle 55 onlyslightly larger than fixture 26.

FIGURES 6 to 9 show a lattice fixture 66 adapted to support the severalparts of the sandwich panel 67 shown in FIG. 10 during brazing. Thispanel comprises core cells 16 of the type previously described, an uppermetal sheet or skin 17 which is to be brazed to the upper end of thecells and a lower sheet 18 to be brazed to their lower end, the planesthrough the ends of the cells tapering toward a tapered bar 68 which isrecessed to provide faces 69 to which the ends of sheets 17, 18 arebrazed. The end of the cell core opposite bar 68 is brazed to thevertical web 70 of a channel 71 Whose outwardly extending legs 72 arebrazed to a pair of elongated doublers 73 which also overlie the cellcore and are brazed thereto and also to the sheets 17, 18. Brazed to theends of sheets 17, 18 and extending past the ends of the cell core arefour strips 74 which serve to distribute any applied load over a largerarea of the cell core, the ends of these strips also being brazed to bar68.

Fixture 66 is a lattice having longitudinal strips 28 and transversestrips 29 which are interlocked in the manner above described, thefixture having an upper section 75 whose lower surface is ground fiatpreferably within a tolerance of .001 inch and a lower section 76 whoseupper face is ground flat within the same tolerance. Lower section 76has a pair of angles 34 attached to it in the manner above described forsupporting the fixture in a mutfie. Attached to the rear side of section76 by bolts 77 is an angle iron 78 in which are seated the ends of threealigned locating pins 79 against the upper ends of which the curved endof bar 68 abuts. Secured in angle 78 are a plurality of cylindricalguide pins 80 which enter holes 81 in an angle 82 secured to upperfixture section 75 by bolts 83. Pins 80 have narrow slots 84 to receivewedges 85. At the opposite end of the fixture similar angles '82, 78 areprovided but instead of the locating pins 79 locating blocks 86 of theshape shown in FIGS. 8 and 9 are used, each block having a hole 87through which guide pin 80 passes. These guide pins also pass throughholes in two thin metal sheets, the upper sheet 88 extending over thetop of the sandwich and the lower sheet 89 over its entire bottom face.The sheets 88, '89 function to distribute the pressure in the samemanner as above described in connection with the sheet 50. The uncoveredends 90 (see FIG. 10) of the honeycomb core are restrained byrestraining blocks 91 held in place by pins 92 whose lower ends areattached to bottom fixture section 76 by welding or other means (notshown).

To weld the component parts of the sandwich together, the surfaces to bewelded are treated as above described in connection with FIG. 2, sheet89 placed on lower section 76 and the other parts positioned as shown inFIGS. 6, 7 and 9. If the weight of upper fixture section 75 providessufiicient pressure to hold the parts to be brazed in firm contact witheach other, the wedges 85 need not be used. Otherwise the wedges aredriven to provide sufficient pressure. The fixture and sandwich 6 V arethen placed in a nrufile and the brazing accomplished in the same manneras above described in connection with FIG. 5.

FIGS. 11 and 12. illustrate an embodiment of the invention in whichvacuum and gas pressure are utilized to apply pressure to a sandwichstructure to hold the skins 17 and 18 in contact with the ends of cells16. The upper section 93 of the fixture has intersecting lattice strips94 of the type above described, the lower edges of which rest on aperforated plate 95 whose lower face is ground to the contour of thedesired sandwich 96. Plate 95 rests on the upper skin 17 which is to bebrazed to the upper end of cells 16. The skin 18 to be brazed to thelower end of the cells rests on perforated plate 97 whose top face isground to the curved contour of sandwich 96. The bottom section 98 ofthe fixture is airtight and may be connected to a vacuum pump 99 by avalve 100. Bottom section 98 has attached thereto a vertical plate 101(FIG. 11) which carries a channel 102 in which is disposed a continuousmass of sealant material 103. Upper fixture section 93 has hermeticallysecured thereto a rectangular hollow casing 104 having vertical side andend Walls 105 which extend into the sealant 103. Upper section 93 isguided by a set of vertical pins 106 whose lower ends are secured tobrackets 107 which in turn are attached to the sidewall of fix-. turesection 98. Wall 105 has attached to it a set of plates 108 drilled toreceive and slide along the pins 106, the latter being slotted toreceive the wedges 109. Surrounding the pins 106 are sleeves 110 whoselength is such that they take the weight of the fnrture section 93 andprevent its weight from hearing on sandwich 96. A pair of Z shaped bars111 extend along the entire side edges of core 16 and are brazedthereto, the webs of these bars being pressed against the core by aplurality of screws 1'12 spaced about 6 inches apart. The screws 112 arethreaded into supports 113 which are attached to the sides of plate 97by bolts 114-. The sides of the panel are thus maintained to therequired dimensions.

The bnazing material having been applied to the top of sheet 18 andbottom of sheet 17 and to those faces of 2 bars 111 which contact thesandwich panel, the panel is placed in the fixture, the top portion 93of the fix ture lowered into position and the screws 112 adjusted. Thefixture is then purged with argon gas and placed in a brazing furnace ofthe type above described. Valve 100 is then opened to apply vacuum tothe interior of fixture section 98 and the bottom face of skin 18,thereby drawing this skin into contact with plate 97 and causing it toconform to the contour of its top face. Simultaneously dry hydrogen gasunder pressure is conducted into hollow casing 104 by a pipe 115 leadingfrom a tank 116 of dny hydrogen gas. This gas reaches the top face ofskin 17 and presses the skin firmly against core '16 and causes the coreto conform to the contour of its bottom skin 18 so that the entire panelhas the desired size and contour prior to reaching the brazingtemperature in the furnace and during the brazing. When the brazing iscomplete, the fixture and panel are removed from the furnace and thevacuum maintained under sheet 18 and the hydrogen pressure over sheet17, until the parts have cooled to about 800 F. when the entire fixtureis purged with argon gas and the brazed panel removed therefrom.

In each of the illustrated examples, in order to prevent the accidentalbrazing of the fixture to the honeycomb panel, those pants of thefixture which contact the panel are covered with a thin coating ofchromium oxide or aluminum oxide (A1 0 The entire fixture may be platedwith chromium in a known manner and the plated fixture then heated in anoxidizing atmosphere to convert the plating to chromium oxide.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essentialcharacteristicsthereof., The present em: bodirnent of the'invention is therefore to beconsidered as in all respects illustrative and not restrictive, thescope of .the inventionbeing indicated by the appended claims, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

Having thus described the invention, what we claim as new and useful anddesired to secure by Letters Patent is:

'1. The method of brazing together the cellular core and skins of ahoneycomb sandwich which comprises the steps of coating a face of eachskin with brazing alloy; placing the sandwich in a fixture constructedto press the coated skin faces against the ends of the core; placing thefixture and sandwich in a muffle; passing an inert gas into the muffleto remove the air therefrom; placing the mufiie with the sandwichtherein in a furnace heated to a temperature above the melting point ofthe brazing alloy; passing a stream of hydrogen gas into the mufilewhile the sandwich is heating to the brazing temperature; removing themuffle with the sandwich enclosed therein from the furnace; and allowingthe muflie and sandwich to cool to a temperature substantially below themelting point of the brazing alloy.

2. The method of brazing together the cellular core and skins of ametallic honeycomb sandwich which comprises the steps of coating a faceof each skin with brazing alloy; securing the sandwich in a fixtureconstructed to press the coated skin faces against the ends of the core;placing the fixture and sandwich in la muffle; passing a stream of argoninto the mufiie to remove the air therefrom; heating the muflie andsandwich to a temperature above 2 12 F. but lower than the melting pointof the brazing alloy; placing the preheated rnufiie and sandwich in afurnace having a temperature somewhat above the melting point of thebrazing alloy; passing a stream of hydrogen gas into the mufiie whilethe sandwich is heat ing to the brazing temperature; removing the mufilewith the sandwich therein from the furnace; and allowing the muffle andsandwich to cool to a temperature substantially below the melting pointof the brazing alloy while continuing to pass hydrogen gas into themufiie.

3. The brazing methods as set forth in claim 1 and including the furtherstep of coating portions of said fix ture disposed adjacent said brazingalloy with an oxide of a metal of the group consisting of aluminum andchromium, thereby to prevent said melted brazing alloy from becomingattached to said portions of the fixture.

4. The brazing method as set forth in claim 3, said oxide coating stepcomprising the steps of plating a thin layer of chromium on the surfaceof said fixture, and oxidizing the chromium to provide a thin coveringof chromium oxide on the fixture.

References Cited in the file of this patent UNITED STATES PATENTS428,056 Smith May 13, 1890 2,166,998 Morgan July 25, 1939 2,235,965 NessMar. 25, 1941 2,243,979 Reynolds June 3, 1941 2,276,847 Kelley Mar. 17,1942 2,406,310 Agule Aug. 27, 1946

1. THE METHOD OF BRAZING TOGETHER THE CELLULAR CORE AND SKINS OF AHONEYCOMB SANDWICH WHICH COMPRISES THE STEPS OF COATING A FACE OF EACHSKIN WITH BRAZING ALLOY; PLACING THE SANDWICH IN A FIXTURE CONSTRUCTEDTO PRESS THE COATED SKIN FACES AGAINST THE ENDS OF THE CORE; PLACING THEFIXTURE AND SANDWICH IN A MUFFLE; PASSING AN INERT GAS INTO THE MUFFLETO REMOVE THE AIR THEREFROM; PLACING THE MUFFLE WITH THE SANDWICHTHEREIN IN A FURNACE HEATED TO A TEMPERATURE ABOVE THE MELTING POINT OFTHE BRAZING ALLOY; PASSING A STREAM OF HYDROGEN GAS INTO THE MUFFLEWHILE THE SANDWICH IS HEATING TO THE BRAZING TEMPERATURE; REMOVING THEMUFFLE WITH THE SANDWICH ENCLOSED THEREIN FROM THE FURNACE; AND ALLOWINGTHE MUFFLE AND SANDWICH TO COOL TO A TEMPERATURE SUBSTANTIALLY BELOW THEMELTING POINT OF THE BRAZING ALLOY.